Fired heater and flue gas tunnel therefor

ABSTRACT

An improved fired heater having one or more flue gas tunnels installed in the firebox floor such that at least most of the lateral cross section of the flue gas tunnel is below the firebox floor.

BACKGROUND OF THE INVENTION

Hydrogen reformer units are commonly used in the petrochemical industry to produce hydrogen gas from methane and steam. One common hydrogen reformer process employs a prior art reformer heater 2 of the type depicted in FIGS. 1 and 2. The prior art heater 2 comprises: a square or rectangular radiant firebox 4; a plurality of rows 6 of process tubes 8 which extend through the radiant firebox 4 and are filled with a catalyst; a collection manifold system 10 which receives the hot process product from the lower ends of the process tubes 8; a plurality of rows of downwardly firing burners 12 installed in the top of the radiant firebox 4 between the rows 6 of tubes 8; a plurality of flue gas tunnels 14 which are constructed within the firebox 4 and extend along the floor 16 of the firebox 4 between each adjacent pair of tube rows 8 and between the outermost tube rows 8 and the side walls of the firebox 4; a plurality of inlet openings 18 through the vertical side walls 20 of the tunnels 14 for continuously receiving the flue gas produced in the firebox 4; an external convection section 22 for recovering heat from the hot flue gas; and exterior flue gas ducts 24 for delivering the hot flue gas from the longitudinal discharge ends 26 of the flue gas tunnels 14 to the convection section 22.

In the prior art top-fired heater 2, the flue gas tunnels 14 are located entirely within the radiant firebox 4 and typically extend upwardly from the firebox floor 16 to a height of as much as seven feet or more. The prior art flue gas tunnels 14 must be constructed within the firebox 4 and have a rectangular cross-sectional shape. The side walls 20 of the prior art tunnels 14 are constructed of firebrick and the flat tops 28 of the tunnels 14 are formed of lintels or tiles which bridge between the upper ends of the side walls 20. In order to encourage uniform flow of the flue gas within the firebox 4 and to assist in obtaining a more uniform collection of flue gas in the tunnels 14 along their entire length, the flue gas openings 18 provided in the vertical side walls 20 of the tunnels 14 are smallest adjacent the discharge ends 26 of the tunnels 14 and progressively increase in size (i.e., cross-sectional area) toward the opposite longitudinal ends 30 of the tunnels 14.

Unfortunately, the flue gas tunnel systems heretofore used in the art have significant shortcomings and are costly and time-consuming to install, maintain, and repair. When constructing the prior art heater 2, several experienced bricklayers are required to erect the vertical tunnel side walls 20 within the firebox 4. In addition, the construction process is very time consuming and difficult due to the extremely limited space and maneuverability within the firebox 4 between the tube rows 6. In addition, the flat lintel or tile covers 28 of the prior art tunnels 14 undergo significant fatigue when exposed to the extreme temperature conditions within the firebox over time and tend to break. This results in the formation of cracks and holes in the covers 28 which further degrade the flue gas flow distribution and uniformity within the firebox 4 and within the tunnels 14. Also, the presence of the tunnels 14 within the already limited space between the tube rows 6 makes it difficult to construct scaffolding within the firebox 4 and to move materials into and out of the firebox 4 for inspecting and repairing the burners 12, tubes 8, and other internal components of the heater 2.

The present invention satisfies the needs and alleviates the problems discussed above. The present invention is well suited for use in both new and many existing top-fired reformer heaters and in any other type of fired heater, whether top-fired, side-fired, bottom-fired, or otherwise, which employs a flue gas tunnel system.

In one aspect, the present invention provides an improvement for a fired heater of the type comprising: a radiant firebox having an interior floor; a plurality of rows of process tubes in the firebox; and at least one flue gas tunnel which extends adjacent to at least one of the rows of tubes and has a plurality of inlet openings for receiving a flue gas from the radiant firebox. The improvement comprises the flue gas tunnel being installed such that at least most of the lateral cross section of the flue gas tunnel is below the interior floor.

In a further aspect of the present invention, the improvement preferably also comprises (a) the flue gas tunnel having a longitudinal top cover and (b) the inlet openings of the flue gas tunnel being provided through the longitudinal top cover. In addition, the longitudinal top cover preferably comprises a series of lateral brick arches in the floor of the firebox with the inlet openings preferably being laterally extending gaps in the longitudinal top cover between adjacent pairs of the lateral brick arches. The lateral slots also preferably extend through the longitudinal top cover at an angle toward a flue gas outlet end of the flue gas tunnel.

In yet another aspect, the improvement preferably further comprises: the flue gas tunnel having a first longitudinal end and a second longitudinal end opposite the first longitudinal end; the second longitudinal end being the discharge end of the flue gas tunnel; and the lateral cross section of the flue gas tunnel increasing in size from the first end to the second end.

The present invention provides several benefits and advantages over the flue gas tunnels heretofore used in the art. Examples of the benefits and advantages provided by the present invention include: eliminating the presence of and the need to construct the vertical brick side walls of the prior art tunnels within the radiant firebox; the ability to construct the flue gas tunnels offsite and install refractory linings therein prior to shipment; significantly reducing the weight of the flue gas tunnels; allowing the addition of convenient man way doors to the firebox for workers and materials; reducing the time required to erect scaffolding within the firebox for inspection and repairs; greatly increasing the available work space and maneuverability within the firebox; significantly reducing the structural support requirements and costs of the heater; and significantly improving the distribution and uniformity of the flue gas flow within the firebox and the flue gas tunnels.

Further aspects, features, and advantages of the present invention will be apparent to those in the art upon examining the accompanying drawings and upon reading the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway elevational side view of a prior art reformer heater 2.

FIG. 2 is an internal perspective view of the prior art reformer heater 2.

FIG. 3 is a partially cutaway elevational side view of an embodiment 50 of the improved fired heater provided by the present invention utilizing an embodiment 52 of the inventive flue gas tunnel having a constant cross-sectional flow area.

FIG. 4 is an internal perspective view of the inventive fired heater 50.

FIG. 5 is a cutaway front elevational view of the inventive flue gas tunnel 52 employed in fired heater 50.

FIG. 6 is a cutaway elevational side view of the inventive flue gas tunnel 52.

FIG. 7 is a partially cutaway elevational side view of an embodiment 80 of the improved fired heater utilizing an embodiment 84 of the inventive flue gas tunnel. The cross-sectional area of the tunnel 84 increases toward the discharge end 86 so that a more constant flow velocity is provided along the length of tunnel 84.

FIG. 8 is a cutaway front elevational view of the inventive flue gas tunnel 84 as seen from perspective 8-8 shown in FIG. 7.

FIG. 9 is a cutaway elevational side view of a segment of the inventive flue gas tunnel 84.

FIG. 10 graphically illustrates the distribution of flue gas inlet flow for the side wall openings of the flue gas tunnels 14 employed in the prior art heater 2.

FIG. 11 graphically illustrates the distribution of flue gas inlet flow for the top openings of the inventive flue gas tunnels 84 of the inventive fired heater 80.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An improved heater 50 employing a first embodiment of the flue gas tunnel system provided by the present invention is depicted in FIGS. 3-6. The improved heater 50 is substantially identical to the prior art heater 2 described above except that the flue gas tunnels 14 of the prior art heater 2 have been replaced with inventive flue gas tunnels 52 which are installed in and extend below the interior floor 54 of the heater firebox 56. The inventive flue gas tunnels 52 extend along the firebox floor 54 adjacent to the rows 58 of tubes 60 in the firebox 56. Although the tube rows 58 shown in FIGS. 3 and 4 constitute horizontal rows of vertical tubes 60, it will be understood that the tube rows can alternatively be vertical rows of horizontal tubes, upwardly angled rows of horizontal tubes, or any other tube row arrangement used in the art.

In contrast to the prior art flue gas tunnels 14, each of the inventive flue gas tunnels 52 preferably comprises: an outer metal casing 62 which is attached to the bottom of the firebox 56 and includes downwardly extending side walls 64 and a bottom 66; a layer of refractory insulation 68 applied on the interior surfaces of the side walls and bottom of the metal casing 62; an interior covering of refractory material (e.g., refractory blocks) 70 installed over the insulation layer 68; a longitudinal top cover 72 installed in the firebox floor 54; and a plurality if inlet openings 74 for receiving flue gas from the firebox 56. Consequently, at least most of the lateral cross section 55 of the inventive tunnel 52 is below the interior floor 54 of the firebox 56.

The flue gas inlet openings 74 can be of any desired type or shape and can be provided through any desired portion of the inventive tunnel which is located in the firebox 56. The openings 74 will preferably be provided through the longitudinal top cover 72 and will most preferably be slots provided in the top cover 72 as shown in FIGS. 3-6 and as described below.

The longitudinal top cover 72 of the inventive tunnel 52 can be a flat cover formed from lintels, tiles, or other materials but will preferably be a laterally arched cover as depicted in FIGS. 4-6. The top cover 72 will most preferably be formed of a series of lateral, sprung firebrick arches 75.

The slots or other openings employed in the longitudinal top 72 of the inventive tunnel 52 will preferably be of sufficient size and number that from about 5% to about 50%, and more preferably from about 10% to about 35%, of the total area of the top cover 72 is open for flue gas flow. The flue gas inlet openings 74 in the cover 72 are preferably laterally extending slots. The laterally extending slots 74 are preferably formed by providing gaps between adjacent pairs of the lateral brick arches 75 at desired locations along the length of the longitudinal top cover 72. The widths of slots 74 will preferably be in the range of from about 1 to about 8 inches. The widths of the slots 74 will more preferably be in the range of from about 1.5 inches to about 6 inches and will most preferably be in the range of from about 2 to about 4 inches. In addition, the lateral flue gas inlet slots 74 can extend vertically through the longitudinal top cover 72 or can extend through the longitudinal top cover 72 at an angle, as described below, toward the discharge end 76 of the tunnel 52.

Another improved heater 80 employing a second embodiment of the inventive flue gas tunnel system is depicted in FIGS. 7-9. The improved heater 80 is essentially identical to the improved heater 50 except that the cross-sectional flow area of each of the inventive flue gas tunnels 84 employed in the improved heater 80 increases in size toward the flue gas discharge end 86 of the tunnel 84. The increase in cross-sectional flow area toward the discharge end 86 operates to provide a more uniform (preferably a reasonably constant) flue gas flow velocity along the length of the tunnel 84.

The increase in the cross-sectional flow area of the inventive tunnel 84 will preferably be such that the cross-sectional flow area of the tunnel 84 at any given point will at least equal, and will more preferably exceed, the total area of all of the top inlet openings 85 and/or other openings in the tunnel 84 leading up to that point. The increase in cross-sectional flow area will also preferably be substantially constant from the first end 88 of the tunnel 84 to the tunnel discharge end 86. Further, although any desired shape can be used, the inventive flue gas tunnel 84 will preferably be of constant width but will have a bottom 90 which slopes downwardly toward the discharge end 86. Typically, the bottom 90 of the inventive flue gas tunnel 84 will slope downwardly toward the tunnel discharge end 86 at a constant angle in the range of from about 2° to about 10°.

The lateral flue gas inlet slots or other openings 85 of the inventive tunnel 84 can extend vertically (i.e., directly downward) through the longitudinal top cover 92 but will more preferably be angled toward the tunnel discharge end 86. The angle of the flue gas inlet slots 85 will preferably be in the range of from about 20° to about 70° from horizontal. The angle of the flue gas inlet slots 85 will more preferably be in the range of from about 30° to about 60° from horizontal and will most preferably be about 45° from horizontal. As will be understood by those in the art, the ends of the fire bricks used to form the lateral brick arches 94 of the tunnel covers 92 can be cut as needed to provide any desired inlet slot angle.

For either of the inventive flue gas tunnel systems described above, it will also be understood that (a) the cross-sectional shape of the tunnel could alternatively be partially circular or any other shape desired, (b) the flue gas inlet openings can alternatively be holes or any other type of aperture, (c) the upper portion of the inventive tunnel can alternatively extend partially above the interior floor of the firebox, (d) if flue gas inlet openings other than lateral slots are used, such openings can also extend vertically or at an angle through the top cover, and (e) although the inventive tunnels will preferably be used throughout the heater, the inventive tunnels can alternatively be used in combination with other types of tunnels (e.g., in combination with one or more prior art tunnels 14).

EXAMPLE

Simulations were run using computer models of (a) a top-fired reformer heater utilizing the prior art flue gas tunnel system depicted in FIGS. 1 and 2 and (b) a top-fired reformer heater employing the inventive flue gas tunnel system illustrated in FIGS. 7-9. The simulations employed: a standard high Reynolds number turbulence model; a discrete ordinates (S4 for ordinate set) method for computing thermal radiation heat transfer; and the weighted sum of gray gases method to compute the gas absorption coefficient assuming natural gas combustion at 10% excess air. Meshes were prepared using the advanced meshing module of Prostar 3.26. The simulations each employed between 1.1 and 1.8 million cells and each required over 60 CPU-hours of processing time.

The models simulate flue gas flow, convection heat transfer from the flue gas to the heater coils, and radiative heat transfer to the coils from the flue gas and refractory surfaces. The flue gas flow is specified as six feet per second with a temperature of 2350° F. at 25 feet above the floor of the radiant box. The tube metal temperature is specified as 1500° F. and the emissivity of the tubes is specified as 0.85, which is typical for oxidized steels. The refractory surfaces are specified as adiabatic and as having an emissivity of 0.65. The ideal gas law is used for determining flue gas density.

In the model for the prior art heater, the flue gas tunnels within the firebox have five openings on each side which are modeled as rectangular cuts through the side walls. The openings increase in size from the outlet end of the tunnel toward the opposite end of the tunnel. However, although the openings adjacent to the tunnel outlet are significantly smaller than the openings toward the opposite end of the tunnel, FIG. 10 shows that a significantly greater amount of flue gas is still received through the smaller openings located near the outlet end of the tunnel than is received through the larger openings located closer to the opposite end or the middle of the tunnel. This shows a significant degree of non-uniformity in flue gas flow and other conditions within the prior art system.

The inventive flue gas tunnels employed in this simulation have two inch inlet slots in the tops thereof which are angled at 45° from horizontal and are spaced apart 20 inches from center to center. The inventive tunnels have sloped bottoms which provide a substantially constant flue gas velocity along the length of each tunnel toward the discharge end thereof. Each tunnel includes a total of 31 angled top openings and provides a significantly more uniform, improved flue gas inlet flow distribution along the length of the tunnel as illustrated in FIG. 11.

The results of the simulation further show that 60.2 MMBtu/hr are absorbed in each row of the tubes of the improved heater using the inventive tunnels as opposed to 58.9 MMBtu/hr for the heater employing the prior art tunnels. Further, the flue gas discharge temperature calculated for the inventive system is 1727° F. versus 1740° F. for the prior art system.

Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims. 

1. In a fired heater comprising a radiant firebox having an interior floor, a plurality of rows of process tubes in said firebox, and at least one flue gas tunnel extending adjacent to at least one of said rows of process tubes, said flue gas tunnel having a lateral cross section and a plurality of inlet openings for receiving a flue gas from said radiant firebox, the improvement comprising said flue gas tunnel being installed such that at least most of said lateral cross section of said flue gas tunnel is below said interior floor.
 2. The fired heater of claim 1 wherein the improvement further comprises: said flue gas tunnel having a longitudinal top cover and said inlet openings being provided through said longitudinal top cover.
 3. The fire heater of claim 2 wherein the improvement further comprises said longitudinal top cover being installed in said interior floor.
 4. The fired heater of claim 2 wherein the improvement further comprises said inlet openings being lateral slots provided in said longitudinal top cover.
 5. The fired heater of claim 4 wherein the improvement further comprises said lateral slots extending through said longitudinal top cover at an angle toward a flue gas outlet end of said flue gas tunnel.
 6. The fired heater of claim 5 wherein the improvement further comprises said angle being in the range of from about 20° to about 70° from horizontal.
 7. The fired heater of claim 5 wherein the improvement further comprises said angle being in the range of from about 30° to about 60° from horizontal.
 8. The fired heater of claim 5 wherein the improvement further comprises said angle being about 45° from horizontal.
 9. The fired heater of claim 1 wherein the improvement further comprises: said flue gas tunnel having a first longitudinal end and a second longitudinal end opposite said first longitudinal end; said second longitudinal end being a discharge end of said flue gas tunnel for said flue gas; and said lateral cross section increasing in size from said first longitudinal end to said second longitudinal end.
 10. The fired heater of claim 1 wherein the improvement further comprises: said flue gas tunnel having a longitudinal top cover which is laterally arched and said inlet openings being provided through said longitudinal top cover.
 11. The fired heater of claim 10 wherein the improvement further comprises said longitudinal top cover being installed in said interior floor.
 12. The fired heater of claim 10 wherein the improvement further comprises said longitudinal top cover comprising a series of lateral brick arches.
 13. The fired heater of claim 12 wherein the improvement further comprises said longitudinal top cover being installed in said interior floor.
 14. The fired heater of claim 12 wherein the improvement further comprises said inlet openings being laterally extending gaps in said longitudinal top cover between adjacent pairs of said lateral brick arches.
 15. The fired heater of claim 14 wherein the improvement further comprises said gaps extending through said longitudinal top cover at an angle toward a flue gas outlet end of said flue gas tunnel.
 16. The fired heater of claim 15 wherein the improvement further comprises said angle being in the range of from about 20° to about 70° from horizontal.
 17. The fired heater of claim 15 wherein the improvement further comprises said angle being in the range of from about 30° to about 60° from horizontal.
 18. The fired heater of claim 15 wherein the improvement further comprises said angle being about 45° from horizontal.
 19. The fired heater of claim 12 wherein the improvement further comprises: said flue gas tunnel having a first longitudinal end and a second longitudinal end opposite said first longitudinal end; said second longitudinal end being a discharge end of said flue gas tunnel for said flue gas; and said lateral cross section increasing in size from said first longitudinal end to said second longitudinal end.
 20. In a fired heater comprising a radiant firebox having an interior floor, a plurality of rows of process tubes in said firebox, and a plurality of flue gas tunnels, each of said flue gas tunnels extending adjacent to at least one of said rows of process tubes and each of said flue gas tunnels having a lateral cross section and a plurality of inlet openings for receiving a flue gas from said radiant firebox, the improvement comprising each of said flue gas tunnels being installed such that at least most of said lateral cross section of each of said flue gas tunnels is below said interior floor.
 21. The fired heater of claim 20 wherein the improvement further comprises: each of said flue gas tunnels having a longitudinal top cover and said inlet openings of each said flue gas tunnels being provided through said longitudinal top cover thereof.
 22. The fired heater of claim 21 wherein, for each of said flue gas tunnels, the improvement further comprises said longitudinal top cover being installed in said interior floor.
 23. The fired heater of claim 21 wherein, for each of said flue gas tunnels, the improvement further comprises said inlet openings being lateral slots provided in said longitudinal top cover.
 24. The fired heater of claim 23 wherein the improvement further comprises, for each of said flue gas tunnels, said lateral slots extending through said longitudinal top cover at an angle toward a flue gas outlet end of said flue gas tunnel.
 25. The fired heater of claim 24 wherein the improvement further comprises said angle being in the range of from about 20° to about 70° from horizontal.
 26. The fired heater of claim 24 wherein the improvement further comprises said angle being in the range of from about 30° to about 60° from horizontal.
 27. The fired heater of claim 24 wherein the improvement further comprises said angle being about 45° from horizontal.
 28. The fired heater of claim 20 wherein, for each of said flue gas tunnels, the improvement further comprises: said flue gas tunnel having a first longitudinal end and a second longitudinal end opposite said first longitudinal end; said second longitudinal end being a discharge end of said flue gas tunnel for said flue gas; and said lateral cross-section increasing in size from said first longitudinal end to said second longitudinal end.
 29. The fired heater of claim 20 wherein, for each of said flue gas tunnels, the improvement further comprises: said flue gas tunnel having a longitudinal top cover which is laterally arched and said inlet openings being provided through said longitudinal top cover.
 30. The fired heater of claim 29 wherein the improvement further comprises said longitudinal top cover of each of each of said flue gas tunnels being installed in said interior floor.
 31. The fired heater of claim 29 wherein the improvement further comprises said longitudinal top cover of each of said flue gas tunnels comprising a series of lateral brick arches.
 32. The fired heater of claim 31 wherein the improvement further comprises said longitudinal top cover of each of said flue gas tunnels being installed in said interior floor.
 33. The fired heater of claim 31 wherein, for each of said flue gas tunnels, the improvement further comprises said inlet openings being laterally extending gaps in said longitudinal top cover between adjacent pairs of said lateral brick arches.
 34. The fired heater of claim 33 wherein, for each of said flue gas tunnels, the improvement further comprises said gaps extending through said longitudinal top cover at an angle toward a flue gas outlet end of said flue gas tunnel.
 35. The fired heater of claim 34 wherein the improvement further comprises said angle being in the range of from about 20° to about 70° from horizontal.
 36. The fired heater of claim 34 wherein the improvement further comprises said angle being in the range of from about 30° to about 60° from horizontal.
 37. The fired heater of claim 34 wherein the improvement further comprises said angle being about 45° from horizontal.
 38. The fired heater of claim 31 wherein, for each of said flue gas tunnels, the improvement further comprises: said flue gas tunnel having a first longitudinal end and a second longitudinal end opposite said first longitudinal end; said second longitudinal end being a discharge end of said flue gas tunnel for said flue gas; and said lateral cross-section increasing in size from said first longitudinal end to said second longitudinal end. 